If the wavelength is big compared to the distance to the rear wall then there should be an addition with the sound that comes back from the wall, which should be a 3db boost if i am right. when the distance to the wall is 1/4 wavelength you get a cancelation notch because of the 180 degree phase shift. this notch will occur periodically at higher frequencies each time the reflected sound and the direct sound are of opposing phase (= comb filter). i dont know a program that takes everything into account. the directivity of the speaker should have a great influence on all this. perhaps you can make a simulation in akabak, but that will be a demanding adventure.

There will also be diffraction effects, which each box has to some degree. Additional to that, room modes and reflections will change the response.